1. Field of the Invention
The invention pertains to the field of phased array antennas, and more particularly to phased array antennas capable of adaptively positioning nulls at desired angular locations.
2. Description of the Prior Art
Antennas which automatically respond to an interference environment, in real time, by steering nulls to and reducing sidelobe levels in the directions of the interference, while retaining the desired main beam characteristics are known in the prior art. These systems usually comprise an array of antenna elements and a real time adaptive receiver-processor which adjusts its element weights toward some optimization of output signal-to-noise ratio in accordance with the selected feedback control algorithm. Generally, the adjustments include both amplitude and phase. Techniques exist, however, for reducing sidelobes and providing nulls in prescribed directions with phased array antennas using phase control only. Utilizing these techniques in a multiple jammer environment with a real antenna, for which sidelobe performance is primarily governed by electrical and mechanical tolerances, is a problem of considerable magnitude. Under these conditions, the determination and implementation of the required phase perturbations is an extremely time consuming process that renders such antennas impractical for many applications.
One antenna of the prior art uses multiple auxiliary antenna apertures which are generally distinct from the main antenna aperture. Jamming signals received by the auxiliary antenna apertures are fed into the received channels of the principal aperture by digital or analog Applebaum loops containing amplifiers with electrically controllable gain and phase. Nulling is accomplished by iteratively solving a co-variance matrix having dimensions proportionate to the number of auxiliary antennas. Convergence is rapid and cancellation is effected if the number of jammers N.sub.J does not exceed the number of canceller antenna loops M.sub.L. In this system, the performance degrades substantially when N.sub.J is greater than M.sub.L. Consequently, the cost effectiveness of this approach is extremely sensitive to the jamming threat, since this dictates the number of loops needed. Each loop requires additional antenna real estate and replication of significant portions of the radar receiver.
The jammer-canceller loop limitation is effectively eliminated in a fully adaptive array, the basic theory of which is well known. A full adaptive array utilizes correlation techniques and feedback control to apply amplitude and phase weights commensurate with the radiation environment to each array element to achieve minimum interference. An excessively large number of phase and amplitude variables must be determined, however, for a fully adaptive large array having thousands of elements. Determination of these variables must be accomplished for a relatively small number of conditions to be satisfied, i.e., nulls to be established. To provide the phase and amplitude weights in real time for such adaptive array antennas, large data strains must be processed. With current technology, the hardware required for such processing is of prohibitive complexity.
In an article entitled "Adaptive Sidelobe Nulling Using Digitally Controlled Phase Shifters" appearing in the IEEE Transactions, Antennas and Propagation, Volume AP-24, No. 5, September 1976, Baird & Rassweiler, addressed this fundamental complexity issue when they recognized that adaptive nulling could be effected by phase control only. This technique provides adaptive nulling by trimming the phase shifters which are used to scan and collimate the surveillance beams in accordance with the sensed interference environment. To reduce the quantity of sensing hardware required, a quasi random search routine, based on a priori knowledge of approximate interfering source locations is implemented. This routine includes interative switching of the smaller phase shifter bits and measurement of incident jamming signals. The convergence of the radiation pattern to effect significant jammer suppression is consequently relatively slow and heavily dependent on a number of jammers and the switching speed of the phase shifters.